One of the most promising materials used for protection of corrosion-susceptible stock against ambient medium for electrical insulation of power line connections and for sealing water and gas lines is a heat-shrinkable material. Tubes, sleeves and hoses made from polymer materials and having been extended at temperatures approaching melt crystallization phase are thermally fixed by cooling to be thereafter capable of reducing their dimetions by being heated and shrink-fitted over objects to be insulated. The use of heat-shrinkable tubes can only be efficient if the tubes are preferably capable of radial shrinkage; however, advancement of the tubes along equipment elements of the production lines in the course of expansion inevitably results in unwanted longitudinal extension of the tubes. Also, mechanical resistance forces arising in the course of tube advancement along equipment elements of the production lines contribute to low efficiency of the tube expansion process, rendering it unstable and low-speed.
Known in the art are apparatus for expanding polymer tubes comprising tapered mandrels or drift-plugs to be drawn inside the tubes. In order to reduce mechanical resistance forces caused by friction, the working surface of the drift-plug is defined by rotating bodies (cf. U.S. Pat. No. 3,201,827).
The rotating bodies mounted in sockets fail to adequately reduce longitudinal extension of the tubes, since contacting surfaces of the rotating bodies and those of the sockets give rise to considerable friction forces preventing easy longitudinal advancement of the tube.
Also known are apparatus for expanding heat-shrinkable polymer tubes comprising inlet and outlet chambers coaxially and consecutively arranged for immediate contact in the direction of the tube advancement, the outlet chamber having a larger diameter than that of the inlet chamber, and a means for creating an overpressure inside the polymer tube (cf. U.S. Pat. No. 3,370,112).
These apparatus feature the inlet and outlet chambers in the form of cylinders, the inner diameter thereof corresponding to the outer diameter of the tube prior to and after expansion. Therefore, the tube to be expanded inevitably contacts the cylinder walls, which results in an increase in the forces of friction arising from the tube sliding along the walls of both the inlet and the outlet chambers, thus causing considerably longitudinal extension of the expandable tubes. In addition, application of antifriction materials fails to reduce friction of the heated tube material against the walls of the chambers. The heretofore described prior art apparatus has limited application from the viewpoint of increasing production efficiency of the process of tube expansion because of the well known dependence between the coefficient of friction and the speed of tube sliding, the coefficient increasing with an increase in the speed of sliding.